1. Field of the Invention
The invention relates to a catadioptric projection objective for imaging a pattern arranged in an object plane onto an image plane.
2. Description of the Related Art
Projection objectives of that type are employed on projection exposure systems, in particular wafer scanners or wafer steppers, used for fabricating semiconductor devices and other types of microdevices and serve to project patterns on photomasks or reticles, hereinafter referred to generically as “masks” or “reticles,” onto an object having a photosensitive coating with ultrahigh resolution on a reduced scale.
In order to create even finer structures, it is sought to both increase the image-end numerical aperture (NA) of the projection objective to be involved and employ shorter wavelengths, preferably ultraviolet light with wavelengths less than about 260 nm.
However, there are very few materials, in particular, synthetic quartz glass and crystalline fluorides, that are sufficiently transparent in that wavelength region available for fabricating the optical elements required. Since the Abbe numbers of those materials that are available lie rather close to one another, it is difficult to provide purely refractive systems that are sufficiently well color-corrected (corrected for chromatic aberrations).
In view of the aforementioned problems, catadioptric systems that combine refracting and reflecting elements, i.e., in particular, lenses and mirrors, are primarily employed for configuring high-resolution projection objectives of the aforementioned type.
The high prices of the materials involved and limited availability of crystalline calcium fluoride in sizes large enough for fabricating large lenses represent problems, particularly in the field of microlithography at 157 nm for very large numerical apertures, NA, of, for example, NA=0.80 and larger. Measures that will allow reducing the number and sizes of lenses employed and simultaneously contribute to maintaining, or even improving, imaging fidelity are thus desired.
Catadioptric projection objectives having at least two concave mirrors have been proposed to provide systems with good color correction and moderate lens mass requirements. The patent U.S. Pat. No. 6,600,608 B1 discloses a catadioptric projection objective having a first, purely refractive objective part for imaging a pattern arranged in the object plane of the projection objective into a first intermediate image, a second objective part for imaging the first intermediate image into a second intermediate image and a third objective part for imaging the second intermediate image directly, that is without a further intermediate image, onto the image plane. The second objective part is a catadioptric objective part having a first concave mirror with a central bore and a second concave mirror with a central bore, the concave mirrors having the mirror faces facing each other and defining an intermirror space or catadioptric cavity in between. The first intermediate image is formed within the central bore of the concave mirror next to the object plane, whereas the second intermediate image is formed within the central bore of the concave mirror next to the object plane. The objective has axial symmetry and provides good color correction axially and laterally. However, since the reflecting surfaces of the concave mirrors are interrupted at the bores, the pupil of the system is obscured.
The Patent EP 1 069 448 B1 discloses another catadioptric projection objective having two concave mirrors facing each other. The concave mirrors are part of a first catadioptric objective part imaging the object onto an intermediate image positioned adjacent to a concave mirror. This is the only intermediate image, which is imaged to the image plane by a second, purely refractive objective part. The object as well as the image of the catadioptric imaging system are positioned outside the intermirror space defined by the mirrors facing each other. Similar systems having two concave mirrors, a common straight optical axis and one intermediate image formed by a catadioptric imaging system and positioned besides one of the concave mirrors are disclosed in Japanese patent application JP 2002208551 A and U.S. patent application Ser. No. U.S. 2002/00241 A1.
European patent application EP 1 336 887 (corresponding to U.S. 2004/0130806 A1) discloses catadioptric projection objectives having one common straight optical axis and, in that sequence, a first catadioptric objective part for creating a first intermediate image, a second catadioptric objective part for creating a second intermediate image from the first intermediate image, and a refractive third objective part forming the image from the second intermediate image. Each catadioptric system has two concave mirrors facing each other. The intermediate images lie outside the intermirror spaces defined by the concave mirrors. Concave mirrors are positioned optically near to pupil surfaces closer to pupil surfaces than to the intermediate images of the projection objectives.
In the article “Nikon Projection Lens Update” by T. Matsuyama, T. Ishiyama and Y. Ohmura, presented by B. W. Smith in: Optical Microlithography XVII, Proc. of SPIE 5377.65 (2004) a design example of a catadioptric projection lens is shown, which is a combination of a conventional dioptric DUV system and a 6-mirror EUV catoptric system inserted between lens groups of the DUV system. A first intermediate image is formed behind the third mirror of the catoptric (purely reflective) group upstream of a convex mirror. The second intermediate image is formed by a purely reflective (catoptric) second objective part. The third objective part is purely refractive featuring negative refractive power at a waist of minimum beam diameter within the third objective part for Petzval sum correction.
Japanese patent application JP 2003114387 A and international patent application WO 01/55767 A disclose catadioptric projection objectives having one common straight optical axis, a first catadioptric objective part for forming an intermediate image and a second catadioptric objective part for imaging the intermediate image onto the image plane of this system. Concave and convex mirrors are used in combination.
U.S. provisional application with Ser. No. 60/511,673 filed on Oct. 17, 2003 by the applicant discloses catadioptric projection objectives having very high NA and suitable for immersion lithography at NA>1. In preferred embodiments, exactly three intermediate images are created. A cross-shaped embodiment has a first, refractive objective part creating a first intermediate image from the object, a second, catadioptric objective part for creating a second intermediate image from the first object, a third, catadioptric objective part for creating a third intermediate image from the second intermediate image and a fourth, refractive objective part for imaging the third intermediate image onto the image plane. The catadioptric objective parts each have one concave mirror, and planar folding mirrors are associated therewith. The concave mirrors are facing each other with the concave mirror surfaces. The folding mirrors are arranged in the middle or the intermirror space defined by the concave mirrors. The concave mirrors may be coaxial and the optical axes of the catadioptric parts may be perpendicular or at an angle with respect to the optical axis defined in the refractive imaging systems.
The full disclosure of the documents mentioned above is incorporated into this application by reference.
The article “Camera view finder using tilted concave mirror erecting elements” by D. DeJager, SPIE. Vol. 237 (1980) p. 292-298 discloses camera view finders comprising two concave mirrors as elements of a 1:1 telescopic erecting relay system. The system is designed to image an object at infinity into a real image, which is erect and can be viewed through an eyepiece. Separate optical axes of refractive system parts upstream and downstream of the catoptric relay system are parallel offset to each other. In order to build a system having concave mirrors facing each other mirrors must be tilted. The authors conclude that physically realizable systems of this type have poor image quality.
International patent applications WO 92/05462 and WO 94/06047 and the article “Innovative Wide-Field Binocular Design” in OSA/SPIE Proceedings (1994) pages 389ff disclose catadioptric optical systems especially for binoculars and other viewing instruments designed as in-line system having a single, unfolded optical axis. Some embodiments have a first concave mirror having an object side mirror surface arranged on one side of the optical axis and a second concave mirror having a mirror surface facing the first mirror and arranged on the opposite side of the optical axis such that the surface curvatures of the concave mirrors define and intermirror space. A front refractive group forms a first intermediate image near the first mirror and a second intermediate image is formed outside of the space formed by the two facing mirrors. A narrow field being larger in a horizontal direction than in a vertical direction is arranged off-set to the optical axis. The object side refractive group has a collimated input and the image side refractive group has a collimated output and entrance and exit pupils far from telecentric are formed. The pupil shape is semi-circular unlike pupil surfaces in lithographic projection lenses, which have to be circular and centered on the optical axis.
The PCT application WO 01/044682 A1 discloses catadioptric UV imaging systems for wafer inspection having one concave mirror designed as Mangin mirror.